LPG/NGL Recovery

Twister has been shown to be able to recover significantly more NGL than a turbo-expander and JT system without the use of hydrate inhibition chemicals.

A recent study looked at the performance and benefits of a Twister NGL recovery system compared to a turbo-expander and a Joule-Thomson (JT) system for an onshore domestic gas supply project. In this project associated gas is dehydrated at the central processing facility to 7 lb/MMSCF in a TEG contactor after which the majority of the gas is sent to an LNG liquefaction facility via a long-distance pipeline. The pipeline inlet pressure is 100 barg. The remainder of the gas is sent to the domestic gas grid, which operates at 60 barg. The available pressure drop is utilised to recover valuable NGL components from the gas, without using any hydrate inhibition chemicals. The recovered NGLs are spiked back in the main gas export line and are recovered at the gas pre-treatment section of the LNG plant. The available pressure drop enables an expansion process without the use of a compressor. Whereas the hydrate formation temperature limits the operating temperature in the LTS for a turbo-expander and JT system, the hydrate formation temperature only limits the Twister system at the inlet of the Twister tube. Inside the Twister tube no hydrate formation takes place due to the very short residence time and hydrates in the secondary outlet of the Twister tube are managed in the Hydrate Separator using heat input. In other words, the minimum acceptable temperature in the LTS for a turbo-expander and JT system is more or less the same as the minimum acceptable temperature at the Twister tube inlet. Therefore Twister is able to expand much deeper into the phase envelope and recover significantly more NGL than a turbo- expander and JT system without the use of hydrate inhibition chemicals. This study also showed that the Twister system has a similar simplicity, robustness and order of magnitude project CAPEX to the JT system, which results in a higher NPV.

An independent study to compare the NGL recovery rate of Twister and a JT system has been conducted by Genesis Oil & Gas covering a wide range of water contents, gas compositions and operating conditions. The recovery rates for LPG, condensate and C3+ have been presented in figure 1 and figure 2.

Figure 1: C3+ recovery as function of water content for Twister and JT-LTS

Figure 2: NGL recovery as function of water content for Twister and JT-LTS using a chiller

This study clearly showed that Twister recovers significantly more NGLs down to very low water contents. At the very low water contents the JT system can utilise the maximum possible heat integration without hydrate formation in the LTS improving the NGL recovery rate, but even at these water contents the NGL recovery rate for Twister is still better.
By applying a chilled water and/or a mechanical refrigeration the Twister inlet temperature can be reduced again to just above the hydrate formation temperature. As expected, this results in a further increase in the NGL recovery as is presented in figure 2. The Twister C3+ recovery increases by approximately 10% (from 50-60% range to 60-70% range) when upstream chilling is applied. The JT-LTS system does not benefit from adding a chiller, since its heat integration is limited by the water in the gas. The same results can be expected for non bone-dry gas applications, where Twister is compared to a turbo-expander, as the water content will limit its LTS temperature.